1. Field of the Invention
The present invention is broadly concerned with improved extruders and methods for extrusion processing of comestible products with very low specific mechanical energy (SME) inputs as compared with conventional methods. More particularly, the invention is concerned with extruders having specially configured screws designed to permit addition of very high quantities of steam, so that the amount of SME required to for complete cooking is maritally reduced. The resultant feeds have very high cook values and expansion characteristics.
2. Description of the Prior Art
Extrusion processing of comestible products such as human foods and animal feeds has long been practiced and is a highly developed art. In general terms, food extruders of the single or twin screw variety are employed, having elongated, tubular barrels with inputs adjacent one end thereof and restricted orifice dies at the outlet thereof, and one or two helically flighted, rotatable screws within the barrel. In many instances steam is injected into the barrel during processing, conventionally by means of injectors oriented at a perpendicular angle relative to the longitudinal axis of the barrel. Depending upon the selected extrusion conditions, the final products may be fully or partially cooked, and can have varying degrees of expansion. This requires a total energy input into the materials being processed, which usually has two components, energy derived from steam injection (specific thermal energy, STE) and SME. It is well known in the art that SME input is significantly more expensive than STE input (generally 2-2.5 times more expensive), and accordingly reducing the extent of SME input required to produce a given product would be very attractive from an economic point of view.
Conventional extrusion systems are limited in the amount of steam which can be injected into the extruder barrel, typically no more than about 5% by weight. This in turn means that SME input must be increased to provide the necessary energy input required. Consequently, the extrusion equipment must have a more robust and therefore construction than would otherwise be necessary, the extent of extruder component wear is higher than desirable, and utility costs are increased.
One factor influencing SME inputs in extrusion processes is the flight depth ratio of the extruder screw(s). This is the ratio of the outer diameter of the screw (SD) to the root diameter of the screw (RD). The flight depth ratio largely determines the shear and volume of output from an extruder. Typical cooking extruders in use today have a flight depth ratio in the range of 1.3 to 1.8. Skilled artisans understand that a ratio of 1.3 is too small, in that the screw(s) lack free volume, creates excessive shear inputs and consumes too much power. Similarly, a ratio above 1.8 is deemed too large, in that the screw has too much free volume and which will prevent barrel fill and the ability to achieve a desired cook value. Accordingly, a flight depth ratio of 1.5-1.6 is considered to be the best compromise. For example, the Wenger TX85 twin-screw extruder has a flight depth ratio of 1.574.
Another geometrical consideration is the ratio of the pitch of the extrusion screw(s) to the flight depth ratio. Smaller values of this ratio for a given pitch translate into lower exit temperatures, increased mechanical efficiency (i.e., more product throughput for a given power input), and greater outputs. The Wenger TX85 extruder with 1.5 pitch screws has a pitch/SD/RD of 0.953, and with 0.5 pitch screws, the value is 0.317.
It is also the general practice in the extrusion art to position adjacent, intermeshed twin screws in a close, self-wiping orientation. Any significant axial gap or clearance between the adjacent flighting sections is considered to be detrimental in that it could create dead zones of accumulated product, and also would decrease the extent of product mixing within the extruder barrel. For example, the Wenger TX85 has an axial gap of 0.039 inches between the adjacent flighting, and this is in keeping with the conventional wisdom of extruder design.
Generally, designers of extruders follow these guidelines in order to achieve what is thought to be the best compromise between extruder size and utility costs on the one hand, versus the need to provide fully cooked and expanded products on the other. That is, an extruder can be designed with smaller drives and screw geometries which will minimize SME. However, these types of extruders will be deficient in that cook values will be unacceptably low and significant expansion cannot be achieved. Alternately, an aggressive extruder design can be used, which will assure adequate cook and expansion of products, but this will inevitably result in high shear and SME inputs with resultant higher costs.
There is accordingly a need in the art for improved extruder equipment and methods which achieve the seemingly contradictory goals of low capital and utility costs with reduced SME inputs, while at the same time being capable of producing fully cooked and expanded products of high quality.